This invention relates generally to semiconductor structures and manufacturing methods and more particularly to electrical components adapted for monolithic microwave integrated circuit (MMIC) fabrication.
As is known in the art, it is frequently desirable to fabricate, and electrically interconnect, both passive and active microwave components on a common substrate. Such arrangement is commonly referred to as a monolithic microwave integrated circuit. Typically the components are electrically interconnected with microstrip transmission lines. The microstrip transmission lines include strip conductor circuitry disposed on one surface of the structure and separated from a ground plane conductor by a substrate. The active devices are typically, for example, heterojunction bipolar transistors (HBTs), or field effect transistors (FETs). A common substrate material used is gallium arsenide. Such substrate material is suitable for the passive microstrip transmission line circuitry, the support of passive devices, such as capacitors, and also for formation of single crystal epitaxial layers used to form the active semiconductor region for the HBTs and the FETs.
As is also known in the art, it is sometimes necessary to electrically connect one of the electrodes of the transistors, or one of the pair of electrodes of a capacitor, to the ground plane conductor. One technique used is to form a via hole which passes from a contact pad, or the electrode, being grounded, through both the epitaxial layer and the substrate, down to the underlying portion of the ground plane conductor. The walls of the via hole are covered with a suitable electrical conductor. Because of the thickness of the substrates typically used, however, the surface area of the contact pad must be relatively large. This large pad places limitations on device topology and also increases the capacitance of the circuit with a concomitant reduction in gain.
As is further known in the art, many active devices are formed with interdigitated electrodes. For example, transistors adapted to operate at high frequencies are sometimes formed with finger shaped gate electrodes (or base electrodes) and finger shaped drain electrodes (or collector electrodes). The finger shaped electrodes are disposed in an interdigitated relationship over a surface of a semiconductor body. Source electrodes (or emitter electrodes) are disposed over the surface and are positioned between a pair of the gate electrodes (or base electrodes). The gate electrodes (or base electrodes) are electrically connected, at proximal ends thereof, to a bus disposed on the surface of the semiconductor. Likewise, the drain electrodes (or collector electrodes) are electrically connected, at proximal ends thereof, to a bus disposed on the surface of the semiconductor body. In a FET, for example, there are two techniques used to connect the source electrodes: the first technique uses bridging conductors, sometimes referred to as air-bridges, which have ends connected to a pair of the source electrodes and which span over gate and drain electrodes; the second uses conductors which pass through vias formed through the semiconductor body beneath the source electrodes, the conductors then being electrically connected with a conductor formed over the bottom surface of the semiconductor body. The former technique is described in U.S. Pat. No. 4,456,888, issued Jun. 26, 1984 and the latter technique is discussed in U.S. Pat. No. 3,969,745, issued Jul. 13, 1976.
The air bridge technique results in increased capacitance between the drain and source electrodes (C.sub.ds) and increased inductance (L.sub.s)in the source electrode circuit. Air bridges, depending on how they are formed, may also prevent visual inspection of device channels and gate electrodes of completed chips. When used with semiconductor bodies having thicknesses of one hundred micrometers, the via connection technique, on the other hand, requires relatively large pads. Large pads, as noted above, contribute to reduced gain and place limitations on device topology.